1. Field of the Invention
The present invention relates to driving methods for liquid crystal display devices in order to improve image quality and liquid crystal display devices and electronic devices in which the driving methods are used.
2. Description of the Related Art
In recent years, replacement of display devices in which conventional cathode-ray tubes with liquid crystal display devices are used as well as employment of liquid crystal display devices in miniature electronic devices has been progressing rapidly. Here, liquid crystal display device refers to a display device in which the alignment orientation of liquid crystal molecules is changed by application of a voltage to the liquid crystal molecules which are interposed between substrates and changes in optical characteristics produced thereby are used.
For a typical liquid crystal display device, for example, twisted nematic (TN) liquid crystal display devices can be given. TN display elements have a structure in which a nematic liquid crystal is interposed between two substrates and the major axis of each of the liquid crystal molecules is twisted continuously along 90° between the two substrates. Consequently, the direction of polarization of light incident on the liquid crystal molecules of the display elements in this state comes to be changed 90° along the twist of the liquid crystal molecules.
Here, when a voltage is applied to the liquid crystal molecules, the major axis of each of the liquid crystal molecules can be tilted in the direction of the electric field by application of a voltage greater than or equal to a certain threshold voltage Vth. That is, the condition of the twist of the liquid crystal molecules can be changed from 90°. At this time, the direction of polarization of light incident on the liquid crystal molecules also comes to be changed according to this twist. A TN system is a system in which this principle is used as a light shutter.
By an active matrix driving of the aforementioned TN system, a display device with better video display performance than that with passive matrix driving can be realized. Here, active matrix driving refers to driving of a pixel by use of transistors that are built into each pixel.
By combination of a TN system and active matrix driving in this way, a certain level of performance for the display device is secured. However, when compared to a display device in which a conventional cathode-ray tube is used, this display device is far from being in a state in which an acceptable pixel is obtained. In recent years, in order to improve image quality, development of liquid crystal materials with fast response has been progressing (for an example of this, refer to Patent Document 1). Furthermore, systems such as OCB (bend orientation) systems and IPS systems changing to TN systems are being employed more and more (for an example of this, refer to Patent Document 2).
Moreover, an approach differing from that given above is also being considered. For example, overdriving is such an approach (for an example of this, refer to Patent Document 3). Overdriving is a driving method in which a high voltage is applied briefly in order to improve the response speed of the liquid crystal molecules. The length of time until the desired luminance is reached can be shortened and the video performance is improved thereby.    Patent Document 1:Japanese Published Patent Application No. H5-17408    Patent Document 2:Japanese Published Patent Application No. H7-84254    Patent Document 3:Japanese Published Patent Application No. H7-104715